Along with proper seeding conditions, weed control is very important early in the season for corn to maximize yield potential. The time between the 3-leaf to 8-leaf stage is referred to in corn as the “critical weed-free period.” Corn is a weak competitor when it comes to other plants, and in order to achieve your maximum yield, weed pressure needs to be reduced early in the season. In the following picture, you can see how controlling weeds at the 3-leaf stage made a big difference in yield compared to spraying at the 8-leaf stage. Granted, waiting until the 8-leaf stage allows you to hit a greater number of emerged weeds—and the field therefore looks very clean late in the season—but the yield hit from having the weeds and crop compete is significant.

Weed Control Chart
Stages of weed growth in corn

Chart, data, and photos: M. Cowbrough, OMAFRA

Glyphosate is the most common pre-seed weed control that we see in Western Canada for corn because the majority of corn acres are glyphosate-resistant, and it tolerates our weather fluctuations fairly well. A light frost of -2 or -3 degrees Celsius won’t significantly affect a plant, and spraying can resume the following day if temperatures rise to 8 degrees Celsius for at least two or more hours. It is critical to determine the weeds, their life cycle (e.g., annual, winter annual, perennial, etc.), and current growth stage in order to select your application rate of glyphosate. 

Don’t forget that tank mixing is a crucial part of any herbicide application, and the pre-seed window is a great time to get extra control of many weeds that might become an issue later in the season. Corn is a crop where there are numerous excellent herbicide options including those that will control glyphosate-tolerant weeds. As stated, corn does not compete well with other plants in the field, so early weed control gives plants a great advantage.

If you were not able to apply a pre-seed or pre-emerge herbicide, there are a number of options available for post-emerge/in-crop spraying. Glyphosate is registered for two applications of 0.67L/ac for 1–8 leaf corn or one application of 1.33L/ac for 1–6 leaf corn. The below chart lists all the approved herbicide options available for in-crop applications and can control a variety of weeds. It is always best to consult product labels or company representatives to confirm any information. Here is a link to the Alberta Crop Protection Book as an additional resource. 

Crop Planner

Maizex Western Canada Crop Planner – Corn 2021

There are many growers who use other management techniques to control weeds. Different cultural practices being used include cultivation, narrower row spacings, cover crops, and destroying weed seeds. Using a combination of techniques is a great way to reduce herbicide-resistant weeds and reduce weed escapes in fields.


Danielle MacCallum CCA, P.Ag

Territory Business Manager – Alberta & BC